Don't worry, guys, Ascension is fixing the lore.

Xerath gained more-or-less infinite power, but was barely contained by a bunch of summoners. Now, those summoners don't exist. Xerath has all the power in the universe, but has no reason to destroy it or anything. He's content to just wait until all his foes die from age; he's obviously not a madman.

Meanwhile, Cho'gath, Brand, Syndra, Jax, and Teemo threaten the entire world. They previously were kept in check by Summoners, but those are all gone now.

Enter Xerath. He has the power to contain all these threats, and a reasonable motivation to do so (he doesn't want the world to be eaten/always on fire). So, he simply steps in and contains these problem children.

The upcoming game mode, Ascension, is basically just his inauguration celebration, where everyone gets to have fun in Xerath's New League of Legends.